When monitoring cameras or detection devices are used, it is important that images be picked up with the highest clarity that the ability of the image sensors can realize. However, where there are stains or foreign objects present on the surface of a sensor, it is impossible to pick up clear images, and when images picked up under such photography conditions are processed, malfunctions can be caused in the device.
When images of objects of the same shape are picked up, such as in the case of manufactured products or the like, it is possible to detect stains or foreign objects if a currently picked-up image and a reference image picked up in advance are compared to each other and they are different. However, when images of objects of different shapes are to be picked up, it is impossible to compare the reference image and currently picked-up images. Also, when the same reference images are picked up and are used for comparison periodically in order to detect stains, time and labor are required to pick up the same reference images. Further, when such reference images cannot be picked up, this method cannot be employed.
In the technique for detecting stains and foreign objects present on the surface of a sensor disclosed in Patent Document 1, a mirror is provided in such a manner that the mirror reflecting the surface of the sensor of the monitoring camera is included at a corner of the picked-up image in order to detect stains and foreign objects by processing the picked-up images. This conventional technique increases the cost and size of a device because a mirror has to be provided outside the sensor. Also, this technique reduces the ability of the monitoring camera because a portion of the picked-up image is occupied by the mirror, which is problematic.
In the technique of detecting stains and foreign objects present on the surface of a sensor disclosed in Patent Document 2, a plurality of images are picked up under different photography conditions by driving image pickup elements such as a lens or diaphragm device, and thereby the portion that does not change from among the plurality of images is determined to be a stain or a foreign object. This method requires movable elements such as a lens, a diaphragm element, or the like, which increases cost. Also, the background portion, which does not change from one image to another, may be determined to be a stain by mistake because portions that do not change are considered to be stains, which is problematic.